1. Field of the Invention
This invention relates to a video signal processing apparatus and more particularly to a video signal processing apparatus having a memory which is capable of storing one field portion of a video signal.
2. Description of the Related Art
Among the apparatuses of the above stated kind, a helical scanning type video tape recorder having a so-called high speed search reproduction function (hereinafter referred to as VTR) is popularly known. The following description, therefore, takes up by way of example the VTR of that type.
The VTR is generally arranged to have different azimuth angles for adjacent recording tracks for the purpose of minimizing a cross-talk between the tracks. Therefore, in cases where a rotary head of the VTR is caused to trace the surface of a tape in the direction of intersecting the tracks by allowing the tape to travel at a high speed, it is impossible to obtain any reproduced signal from one of the adjacent tracks. The reproduction output obtained during such a period, therefore, deteriorates and thus causes noise bars to appear on a reproduction image plane.
In one solution of this problem, a pair of heads differing in azimuth angle from each other are adjacently disposed. For example, two pairs of such heads are arranged to revolve at a phase difference of 180 degrees. The noise bars in question is prevented from showing by allowing one of each pair of heads which has the same azimuth angle as that of the currently traced track to produce a reproduced signal.
In another solution, the VTR is provided with a memory which is capable of storing one field portion of a video signal. The memory is arranged to store, during high speed search reproduction, the video signal only when the reproduction output thereof is not deteriorated. When the reproduction output is deteriorated, the deteriorated part of the reproduced video signal is replaced with a corresponding part of the reproduced image plane previously stored in the memory, and such corresponding part is read out. In the case of this method, one field portion of the video signal stored in the memory is obtained by tracing the tape with a rotary head several times. Therefore, in cases where one and the same part of the video signal on the reproduced image plane happens to be continuously deteriorated over a long period of time, the video signal stored in the memory is not renewed for a long period of time. Then, an undeteriorated part of a reproduced signal and the long-time-unrenewed part would become hardly matchable with each other. This would result in a very unnatural reproduced picture.
In view of this, it has been considered to make the travelling speed of the tape an even number times as high as the tape speed employed in recording in case that the above stated memory is used. In this case, the VTR having two heads operates in the following manner: FIGS. 1(a) to 1(e) of the accompanying drawings show this operation. FIG. 1(a) shows the tracing loci of the heads on the tape obtained during reproduction at a tape speed four times as high as the recording tape speed. In FIG. 1(a), crosshatching denotes parts where a reproduced signal is obtained. Reference numerals represent track numbers, i.e. field numbers. FIG. 1(b) shows a head change-over pulse. FIG. 1(c) shows a writing/reading switching pulse for a memory. FIG. 1(d) shows the envelope wave form of the reproduced signal.
The reference numerals of FIG. 1(d) represent field numbers corresponding to those of FIG. 1(a). FIG. 1(e) shows field numbers of images displayed in varied parts of a reproduced image plane. No unnatural image will be displayed as the image is completely renewed in a cycle of two fields.
However, the above stated arrangement of using paired heads requires use of four heads with each pair of them adjacently disposed in the case of a two-head helical scanning type VTR. This necessitates a complex head arrangement. Further, the increase in the number of heads causes the rotary transmitter of the VTR to have many channels. Then, cross-talks between channels increase, and hence this head arrangement has been hardly applicable to a rotary head drum of a small diameter.
In the case of the method of using the memory, there has been the following problem: A rectangular wave signal (hereinafter referred to as 30 PG signal) having two field periods (30 Hz) is obtained by detecting the rotation phase of the rotary head. In the event of reproduction at a tape speed which is n times (n: an integer) as high as a tape speed used for recording, the 30 PG signal is generally stepped up by .vertline.(n-1).vertline. times by means of a PLL or the like and is used for switching between writing and reading actions on the memory. In the case of an interlaced scanning NTSC TV signal or the like, however, the phase of the horizontal synchronizing signal relative to the rotation phase of the rotary head obtained in the first field portion of the signal comes to shift as much as 0.5 horizontal scanning period (hereinafter referred to as H) in the second field portion of the signal. Further, a time-base variation taking place during reproduction causes the horizontal synchronizing signal to become incontinuous when the writing and reading actions on the memory are switched from one over to the other under the control of the above stated stepped up signal. This results in a skew and brings about some adverse effect on a reproduction image plane.
FIGS. 2(a) to 2(g) show in a timing chart the above stated switch-over between writing and reading actions performed on the memory by the conventional VTR. In the case of this illustration, tape is allowed to travel at a speed which is four times as high as the recording speed. FIGS. 2(a), 2(b) and 2(c) respectively show the signals obtained for the first field including the 30 PG signal, a writing/reading switching signal and a composite synchronizing signal included in the video signal. The writing action on the memory is arranged to be performed when the writing/reading switching signal is at low level and the reading action to be performed when this signal is at a high level. FIGS. 2(d), 2(e) and 2(f) likewise respectively show the signals obtained for the second field including the 30 PG signal, the writing/reading switching signal and the composite synchronizing signal. FIG. 2(g) shows a composite synchronizing signal included in the video signal obtained for the first field according to the above stated writing/reading switching signal. As shown, there take place skews at three parts X within one field.
In handling the so-called composite video signal which includes a carrier chrominance signal, switching between the writing and reading actions on the memory must be performed without impairing the continuity of the carrier chrominance signal. To meet this requirement, it has been considered to handle a digitized composite video signal by always handling each cycle of the carrier chrominance signal as one unit. For example, with a video signal sampling frequency assumed to be n fsc (wherein n represents an integer which is at least 3), an n number of sampled data are handled as one unit. However, this method is effective only in cases where the continuity of the color subcarrier of the original analog video signal is retained. It is impossible to ensure the continuity of the color subcarrier stored in the memory in cases where a plurality of video signals are to be written into the memory in a time sharing manner and to be read out from the memory; or where only the acceptable parts of a video signal in each field are to be incontinuously written into the memory during a high speed reproducing operation.
Therefore, in the case of storing in the memory a plurality of video signals being received at different timings as mentioned above, the conventional VTR is arranged to temporarily bring their carrier chrominance signals back into a plurality of base band signals such as color difference signals or the like and to store them separately in different memories. However, this arrangement has been undesirable as it necessitates the use of many discrete memories of large capacities for the luminance signal and two color difference signals,